First I would like to know that by saying in-silico approaches, did u mean target prediction tools or Drug-miRNA binding search tool???
If its target prediction tools then number of tools available in public domain such as miRDB, miRTAR, MirSYSyem, targetScan etc.
or if you are looking for Drug-miRNA binding then try Pharmaco-MiR tool.
I may not know about very specific miRNA and polyphenol interaction specific tool but Pharmaco-miR can work for you. this tool has enough information about drug- miRNA interaction, so any drug which resembles the polyphenol structure could give some idea of miRNA-polyphenol interaction idea.
Another option for you can be molecular docking approach by generating 3D structure of you miRNA and polyphenol and then performing molecular docking, it may provide you structural insights.